THIS invention relates to a diffuser and more particularly to a chain diffuser for use in a sugar extraction process.
A diffuser is an apparatus commonly used in the sugar industry to extract sugar-bearing juice from shredded cane or bagasse. Operation of a diffuser is based on systematic counter current washing of the cane or bagasse by means of imbibition water. This is achieved by forming a bed of cane or bagasse on a conveyor, while copious quantities of water and thin juice is allowed to percolate through the bed to wash out the sugar bearing juice. The water is added at the discharge end of the conveyor and is pumped forward stage by stage towards the feed end, with each stage being defined by a completed percolation cycle. The process is repeated until the juice reaches maximum concentration at the feed end of the diffuser.
A typical diffuser is 60 m long by 6-12 m wide. The floor of the vessel is made of perforated plate, over which a carrier rides. The carrier is in the form of chains with slats (conveyor grids) between two chains which drags the slats through the diffuser. A pair of chains and its slats is commonly known as a ‘chain ladders’ with a width of approximately one to two meters. The drag chains are supported on chain runners (or wear strips) along the length of the diffuser and are driven with sprockets at the headshaft through a common headshaft. The return length of the chains is either supported by return idlers or a return runner. The tailshaft is in the form of a smooth idler.
The chain ladders drag the cane along at a speed of about 1 meter per minute. Under the perforated floor, the diffuser is divided into 10-12 stages, each stage analogous to a single milling unit. Press water and imbibition are added ahead of Stage 12; thin juice from Stage 12 is recycled and added ahead of Stage 11; thin juice from Stage 11 is recycled and added ahead of Stage 10—and so on until the juice is withdrawn from Stage 1. A portion of the Stage 1 juice is tapped off, heated, and poured onto the incoming cane to saturate and heat it. This fraction of juice is called scalding juice. The bulk of the juice is sent to the process stream for further processing.
The discharge end of the diffuser is sealed by a rotating weighted drum which dewaters the cane to some extent. A spiked rotor called a kicker breaks off chunks of the hot fibre that then fall onto a carrier that feeds the dewatering mill.
Percolation through the fibre bed is critical in a diffuser. Batteries of lifting screws disturb the bed, preventing any packing that may occur during its 1-hour travel through the diffuser. A blinded bed results in juice travelling over the top of the fibre, and the diffuser is then said to be flooded. Imbibition rates tend to be higher for a diffuser than for a mill, while similar extractions can be obtained. Where one (or two) conventional mills precede a diffuser, that diffuser is referred to as a bagasse diffuser. A cane diffuser handles prepared cane.
Current diffuser designs utilize a large head shaft and drive to drag the bed of cane through the diffuser. Although this works well from a process perspective, a number of disadvantages are associated with this configuration when viewed from a mechanical and structural perspective. These include:                The load on the head shaft is high and large amounts of steel are required for manufacturing the head shaft;        Foundations are substantial and expensive;        A large and expensive single drive is required;        The drive cannot be viably kept as a stock item, thus resulting in substantial downtime should there be a major failure; and        The width of the diffuser is fixed at the time of manufacture due to the continuous head shaft, and expansion of capacity by widening the diffuser will entail replacement of the entire head shaft and drive, thus rendering expansion commercially not viable.        
To reduce the cost of manufacture a “walking floor diffuser” has been suggested, and is currently used in industry, for example WO2007/015124. The walking floor diffuser utilises an established method for moving bagasse, and comprises a series of strips of perforated plates/panels that all move forward at the rate at which it is desired that the bagasse or cane mat will travel. The plates/panels are moved hydraulically, and this type of diffuser does not use a chain driven system. Plates or panels are rapidly returned to the feed end of the diffuser once the discharge end is reached. In this way the floor slowly moves the bagasse forward. The advantage is that there is no head shaft and drive, and the diffuser is therefore expandable in width by adding more rows of perforated plate/panels. However, there are a number of disadvantages with this particular arrangement. These include:                The bagasse tends to block the perforated plate because the scraping action of the chain and slats are not present;        The hydraulic drive arrangement is complex and expensive to maintain;        Controls are complex and any sequencing problems result in fractures of the plates;        The perforated plates carry the entire weight of the bagasse bed, thus resulting in excessive wear;        The bagasse tends to compact; and        The rapidly returning strip of perforated plate results in juice bypassing the intended counter current flow path, and extraction efficiency is reduced.        
It is accordingly an object of the invention to provide a diffuser that will, at least partially, alleviate the above disadvantages.
It is also an object of the invention to provide a diffuser which will be a useful alternative to existing diffusers.